Effects of fasting on metabolic and immunological responses of common carp ( Cyprinus carpio ) to a further acute stress

Common carp were fasted for 0, 3 and 14 days prior to exposure to a 2‐hr crowding stress. Serum cortisol, glucose, lactate, T₄, T₃, serum alternative complement (ACH50), lysozyme, total immunoglobulin (Ig) and blood WBC were determined before and 0, 6 and 24 hr after the crowding stress. Three days of fasting led to a significant decrease in serum levels of lactate and thyroid hormones. Fourteen days of fasting led to initiation of physiological process to maintain serum glucose by increase in serum cortisol level and lactate utilization. These fish had lower levels of thyroid hormones suggesting suppression of basal metabolism. The fish also had lower WBC and serum lysozyme and total Ig levels indicating immunosuppression. After the crowding stress, the fish fasted for 3 days had significantly lower lactate, thyroid hormones and total Ig levels. The fish fasted for 14 days had higher cortisol but lower glucose and thyroid hormones levels suggesting impairment in metabolic responses to the crowding stress. Immunological and health indicators including WBC, and ACH50, lysozyme and total Ig of these fish were significantly lower than the other groups after the crowding stress. Overall, the results show that a 14‐day fasting leads to stress responses, reduction in metabolism and immunosuppression. Moreover, 14 days of fasting lead to greater cortisol response to a further crowding stress, which induces marked immunosuppression and fallen welfare.